Motor-driven power steering unit

ABSTRACT

The invention relates to a motor-driven power steering unit, comprising an electric motor for producing an auxiliary steering force in a steering system that couples a steering handle to the wheels of a vehicle. The power steering unit assists the rotation of the steering wheel by controlling an electric current fed to the electric motor based on steering information. Said unit also comprises a steering angle detection device and an overload protection device to reduce the electric current fed to the electric motor, wherein the overload protection device reduces the electric current fed to the electric motor if the steering angle detected exceeds a predetermined angle.

[0001] The present invention relates to a motor-driven power steering unit with an electric motor for producing an auxiliary steering force in a steering system that couples a steering handle to the wheels of a vehicle, whereby the power steering unit assists the rotation of the steering wheel by controlling an electric current fed to the electric motor based on steering information comprising a steering angle detection device.

[0002] A generic motor-driven power steering unit is known from DE 38 21 789. This power steering unit comprises a steering force protection device for detecting the steering force produced in the steering system as well as an overload protection device to reduce the electric current fed to the electric motor. On the basis of the steering force detected, which is measured, for example, on the torsion bar of the steering system, the support is reduced by means of the overload protection device if the steering force detected exceeds a pre-determined angle. In addition, DE 38 21 789 discloses a generic power steering unit, in which additionally the steering speed is measured and this variable is also considered in the computation or reduction of the support. With this embodiment, the support is only reduced if, on the one hand, the hand force level is exceeded and, on the other hand, the steering speed has not exceeded a certain value. The power steering units known from DE 38 21 789 have the disadvantage that the driving feel at the end stop is determined by the rotational rigidity of the torsion bar of the steering system and cannot be influenced or regulated by controlling the power steering unit.

[0003] Object of the present invention is to create a power steering unit, in which the driving feel within range of the left and right end stop can be influenced in a pre-determinable way i.e. by a corresponding reduction of the support.

[0004] This object is achieved according to the invention in each case by means of a motor-driven power steering unit with the features of the two independent claims 1 and 2. Sub-claims 3 to 16 contain further features, which additionally form the power steering units of claims 1 and 2.

[0005] The power steering unit according to the invention detailed in the two independent claims 1 and 2 are advantageously characterized in that use of steering angle information already available in any modern steering system can be made. Advantageously, the support or the current to be supplied, which has been detected beforehand by the motor control, can already be reduced before the respective end stop is reached. The extent of the reduction in this case advantageously depends on the steering angle, so that the support can be reduced more and more the nearer the respective end stop is reached.

[0006] Advantageously, the reduction can result, for example, due to the fact that the value detected by the motor control for the current to be supplied is multiplied by a proportionality factor a and then the value is transmitted to the controllable power source, which then regulates the current to be supplied. The function a=f (steering angle) in this case can be selected at will. If this support is to be prevented from being totally reduced, that is to say to zero, it is advantageous to specify a lower minimum value for the proportionality factor.

[0007] It is also advantageous that the reduction of the support only becomes active if the vehicle has not yet exceeded a certain speed. Thereby, erroneous detection of the steering angle only having an effect at low speeds is prevented and no disturbance arises at higher speeds through inadvertent influence of the support.

[0008] Likewise the range, within which the support is to be reduced, can be freely specified by pre-setting limit angles independently of technical parameters.

[0009] So that the steered vehicle wheels can be turned with the full support out of one of the two end positions to the central position, the power steering units according to the invention possess a function, which query the signs of the hand force and the steering angle. In this case, depending on the sign convention for each end position two cases are differentiated. If both variables have the same sign, which means that the steering force is directed in such a manner that attempt is made to move the wheels toward the end position, the support is reduced, if the aforementioned limit angle is exceeded. When the wheels are steered out of the end position toward the central position on the other hand, the signs of the variables are different. In this case, the maximum or optimum support is to be made available to the driver. The proportionality factor in this case therefore is set to one, so that the value computed by the motor control for the current to be supplied is passed onto the variable power source.

[0010] Naturally, the invention is not limited to the fact that the value of the current to be supplied is only multiplied and handed over to the motor control or the controllable power source by means of the proportionality factor. It is also possible that the support force computed by the steering system is multiplied with the proportionality factor and then the product is handed over to the motor control and/or another control or regulating unit.

[0011] The operational principle of the invention is described in detail below on the basis of drawings, wherein:

[0012]FIG. 1 shows angle ranges, in which the support is reduced, if the vehicle speed has not exceeded a certain value;

[0013]FIG. 2 shows a possible function a=f (steering angle);

[0014]FIG. 3 is a flow chart for a possible process to compute a proportionality factor;

[0015]FIG. 4 is a block diagram for a possible arrangement of the steering angle dependent-reduction unit or “endlock protection” unit in the steering system.

[0016]FIG. 1 serves to explain the variables relevant to the power steering unit according to the invention. In this case, “endlock_right” designates the angle, at which the steering system is positioned in the right mechanical end stop. “endlock_left” designates the angle, at which the steering system is positioned in the left mechanical end stop. The angle φ_(gr) is the right limit angle and φ_(gl) is the left limit angle. If the steered wheels are rotated in such a manner that the steering angle falls in the hatched area, the support or the current to be supplied is reduced. If optionally the vehicle speed v_(vehicle) is considered, the support or the current to be supplied is only reduced, if the vehicle has not exceeded a certain preset speed. Depending on the sign convention selected, the steering angle φ_(sw) is positive, if the steering wheel or the wheels are moved to the right. The steering angle φ_(sw) on the other hand is negative, if the steering wheel or the wheels are turned or moves to the left. If the steering wheel and/or the steered vehicle wheels are in the central position, the steering angle φ_(sw) is equal to zero.

[0017]FIG. 2 shows a possible functional correlation between the factor a and the steering angle φ_(sw). The factor a is equal to one, if the steering angle φ_(sw) has not exceeded either of the two limit angles φ_(gr) and φ_(gl). On the other hand, as soon as the steering angle φ_(sw) has exceeded one of the two limit angles φ_(gr) and φ_(gl), the factor a becomes less. If necessary, additional query and limitation for the factor a can result. Thus, for example, the value can be limited to a lower limit value min_support, which it reaches with the steering angle φ_(sw) equal to endlock_right or endlock_left or beforehand. The functional correlation shown in FIG. 2 is only given by way of example. Naturally, it is possible to preset the curve progression differently depending on need. For example, a linear, quadratic, exponential or logarithmic decrease of the factor a in the angle ranges endlock_left<φ_(sw)<φ_(gl) or φ_(gr)<φ_(sw)<endlock_right is conceivable. Thus, any appropriate values for min_support can also be specified.

[0018]FIG. 3 shows a flow chart for a possible process to implement the power steering units according to the invention. In a first step 100, it is checked whether the vehicle speed has exceeded a certain value v_(limit). If the vehicle is travelling at a speed greater than v_(limit), the proportionality factor support_factor is set in step 1000 equal to one, whereby the computation process is continued to the end, and is afterwards restarted. If the vehicle has not exceeded v_(limit) it goes to step 200, where the sign of the steering angle φ_(sw) is checked. If φ_(sw) is less than zero, that is to say turned to the left, the factor a is computed by means of the step 400. The angle difference between the end stop angle endlock_left and the steering angle φ_(sw) is computed in this case. This difference is divided by the angle difference of endlock_left minus φ_(gl). Since the angle difference endlock_left minus the limit angle φ_(gl) is a constant, which indicates an angle range, it can also be divided by this previously computed and stored angle range value. If the last mentioned angle difference is positive, the absolute value of a does not need to be computed. Accordingly, the factor a is computed in step 300, if the steering angle φ_(sw) is greater than zero. The factor a is a function of the steering angle φ_(sw). Depending on the preset limit angle φ_(gl) or φ_(gr), the value of the factor a, in the case of a steering angle φ_(sw) becoming lesser or greater, will fall short of a certain pre-definable value. In step 500, it is checked whether the value previously computed in steps 300 and 400 is greater than one. If, in the case of the process shown in FIG. 3, the value of a is greater than one, the proportionality factor support_factor is set equal to one and the process is ended or the value of the proportionality factor support_factor is passed on for further computation of the support. If the value of the factor a is less or equal to one, the signs are checked in step 600. In this case, alternatively either the direction or the sign of the hand force tq_(sensor) can be compared with the sign of the steering angle. Or the direction or the sign of the support or the controlling signal tq_(reg) already detected by the steering system or controller is compared with the sign of the steering angle. If the signs or directions do not coincide, it goes to step 1000 and the proportionality factor support_factor is set equal to one. If the signs of the values are unequal, the case results that attempt is made to move the wheels or the steering wheel towards the central position. If the signs are identical, attempt is made to move the wheels or the steering wheel towards the end position. In this case, it is checked in step 700 whether the value of the factor a is less than the minimum permissible support factor min_support. If a<min_support, it goes to step 800 and the proportionality factor support_factor is set to equal min_support. If a>=min_support, it goes to step 900 and the proportionality factor support_factor is set to equal a. After steps 800 and 900 are completed, the computation process is concluded and the computation of a new proportionality factor support_factor can be started.

[0019]FIG. 4 shows a block diagram for a possible arrangement of the steering angle dependent-reduction unit or “endlock protection” unit in the steering system. The controller produces a controlling signal tq_(reg), which is multiplied with the proportionality factor support_factor. The product of the multiplication is then fed to the motor control of the electric motor, which generates the support. The proportionality factor support_factor is computed by means of the “endlock protection” unit, which for example uses the process described in FIG. 3. Only absolutely essential input variable for the “endlock protection” unit is the steering angle φ_(sw). Optionally, the vehicle speed v_(vehicle) and the hand force detected or the size of the adjusting signal tq_(reg) can also be used to determine the proportionality factor support_factor. 

1. Motor-driven power steering unit with an electric motor for producing an auxiliary steering force in a steering system, that couples a steering handle to the wheels of a vehicle, whereby the power steering unit assists the rotation of the steering wheel by controlling an electric current fed to the electric motor based on steering information, comprising a steering angle detection device, characterized in that the power steering unit regulates the support depending on the steering angle detected.
 2. Motor-driven power steering unit with an electric motor for producing an auxiliary steering force in a steering system, that couples a steering handle to the wheels of a vehicle, whereby the power steering unit assists the rotation of the steering wheel by controlling an electric current fed to the electric motor based pn steering information, comprising a steering angle detection device as well as an overload protection device to reduce the electric current fed to the electric motor, characterized in that the overload protection device reduces the electric current fed to the electric motor if the steering angle detected exceeds a preset angle.
 3. Motor-driven power steering unit according to claim 1 or 2, characterized in that the support or the electric current is reduced in the angle ranges before the respective end positions of the steered vehicle wheels are reached.
 4. Motor-driven power steering unit according to any one of the above claims, characterized in that the support or the current is reduced after exceeding or after reaching a preset or pre-definable angle before reaching the respective end position.
 5. Motor-driven power steering unit according to any one of the above claims, characterized in that the support is reduced in such a manner that the steered wheels or the steering handle or the steering rod can at maximum only reach the end positions with a pre-set maximum speed and/or force.
 6. Motor-driven power steering unit according to any one of the above claims, characterized in that the support is only reduced if the vehicle has not exceeded a certain speed.
 7. Motor-driven power steering unit according to any one of the above claims characterized in that the reduction of the support or the current is increased as soon as the signs of steering force and steering angle no longer coincide.
 8. Motor-driven power steering unit according to any one of the above claims characterized in that the support is continually reduced more and more after a certain pre-definable or pre-set angle φ_(gr) or φ_(gl) is reached until the respective end position is reached.
 9. Motor-driven power steering unit according to claim 8, characterized in that the reduction increases linearly, exponentially, quadratically or corresponding to an angle dependent-function.
 10. Motor-driven power steering unit according to any one of the above claims, characterized in that the support computed by the steering system or the value of the electric current to be supplied is multiplied with a proportionality factor “support_factor”, and the electric motor is controlled based on the support resulting from this or the electric current resulting from this is fed to the electric motor.
 11. Method to compute the proportionality factor “support_factor” for a motor-driven power steering unit according to claim 10, characterized in that it is checked in a first process step whether the actual vehicle speed v_(vehicle) has exceeded a certain speed v_(limit), if v_(vehicle)>=v_(limit), the proportionality factor “support_factor” is set to equal 1 (one) and the process ends, if v_(vehicle)<v_(limit), the computation of the proportionality factor “support_factor” is continued.
 12. Method according to claim 11, characterized in that if the condition v_(vehicle)>=v_(limit) is present, it is checked in a subsequent process step whether the steering angle is positive or negative, whereby a factor a is then computed subsequently for the respective angle ranges whereby $a = {\frac{\phi_{s\quad w} - {endlock\_ left}}{{endlock\_ left} - \phi_{g\quad l}}}$

if the steering wheel is turned from the central position to the left, and whereby $a = {\frac{{endlock\_ right} - \phi_{s\quad w}}{{endlock\_ right} - \phi_{gl}}}$

if the steering wheel is turned from the central position to the right, whereby endlock_right is the right stop angle, endlock_left is the left stop angle and φ_(sw) is the actual steering wheel angle.
 13. Method according to claim 12, characterized in that for values of the factor a, which are greater than the value one, the proportionality factor support_factor is set equal to one and the computation is ended.
 14. Method according to claim 12 or 13, characterized in that the proportionality factor support_factor is set equal to the factor a, if the value of the factor a is greater than or equal to a pre-set minimum value min_support and less than or equal to one and that the proportionality factor support_factor is set equal to the minimum value min_support, if the factor a is less than the minimum value min_support.
 15. Method according to any one of claims 11 to 14, characterized in that the proportionality factor support_factor is set equal to one, as soon as the signs of steering force and steering angle no longer coincide. 